Electroplating on aluminum



Patented Sept. 1, 1953 UNITED STATES PATENT OFFICE ELECTROPLATIN G ON ALUMINUM No Drawing. Application June 8, 1949,

Serial No. 97,912

7 Claims. (Cl. 204--33) This invention relates to electroplating on alu-= minum, and more particularly to the preparation of aluminum articles for the electroplating of a metal thereon and especially the electroplating of chromium on the aluminum.

The invention is applicable not only to aluminum itself as the base or foundation metal,

such as the product of the Aluminum Corpora-v tion of America known as 28 which is sold as commercially pure aluminum and of which some 99% or more is aluminum metal, but also to common commercial aluminum alloys as well, i. e. those alloys which are predominantly aluminum; for example, such aluminum alloys of the Aluminum Corporation of America as the wrought aluminum alloys identified as 248 (having 4.5% copper, 1.5% magnesium, and 0.6% manganese), and 7553 (1.6% copper, 2.5% magnesium, 5.6% zinc, and 0.3% chromium); the die casting aluniinum alloy identified as 380 (having 3.5% copper and 9.0% silicon) and the sand casting alloys identified as 355 (1.3% copper, 5.0% silicon, and 0.5% magnesium) and 356 (7% silicon and 0.3% magnesium). Accordingly in this specification and its claims the word aluminum is used in a generic sense, i. e. as including both pure aluminum and those alloys which are predominantly aluminum, except as the contrary may appear.

Heretofore various methods have been proposed for plating on aluminum. Generally at least, each of these includes a series of steps for preparing the aluminum article for the subsequent electrodeposition of the desired metal. In some instances the preparatory steps include an alkaline-etch cleaning operation at an elevated temperature, around 180 F. or more, the purpose of which is to remove the layer of oxide which occurs naturally on all commercial aluminum. Also where chromium is to be electrodeposited on aluminum, at least one plating method used heretofore includes, as a final preparatory step, the electrodeposition of copper on the aluminum, evidently to serve as a bonding metal between the aluminum base and the chromium plate. All the prior practices, so far as I am aware, have undesirable features, and in particular the obtaining of a bond between aluminum and chromium adequate to permit the use of a chromium plate for wear resistance, is, in repetitive factory practice, somewhat difficult and uncertain, or is expensive.

The present invention provides for plating on aluminum with a strong adhesion or bond between the plate and the aluminum surface under production conditions, e. g. a bond sufiicient to permit the electroplating of chromium on alu- 'minum for wear resistance purposes. Also in contrast to at least some prior processes, the invention reduces the number of steps preceding the actual electrodeposition of the desired plating metal; for example, it is not necessary, with the invention, to plate the aluminum with copper before depositing chromium.

The invention secures these results by a certain distinctive operation or operations preparatory to the electrodeposition of the metal on the aluminum base metal. Basically this or these preparatory operations comprise subjecting the aluminum or such area or areas of the aluminum as are to be plated, and preferably after having degreased or otherwise cleansed the area or areas to be plated, to vapor blasting until the adherent film of oxide that is found on commercial aluminum has been thoroughly broken down, and preferably, in addition, a fine roughness imparted to or developed on the aluminum surface, or subjecting the area or areas to be plated to another physical treatment, or as it may be called, mechanical treatment, that yields the same results, and also, and advantageously following such physical treatment, subjecting the area or areas to the action of a solution containing caustic alkali and alkali zincate. These primary or basic treatments alone are sufiicient to permit the ready plating with a strong bond on some of the common aluminum alloys, or some of the alloys under certain conditions, and may be followed directly by the electrodeposition of the desired metal in, say, the ordinary way. However by adding to these primary or basic treatments an additional step consisting of subjecting the area or areas to be plated to a solution or solutions of a strong mineral acid or acids, e. g. nitric or both sulphuric and nitric, following the physical treatment and prior to the alkali and zincate treatment, not only is the process adapted for plating on the same alloys as before, and the same alloys under the same conditions, but also the process is adapted for plating on still other alloys. Also by adding a still further treatment, 1. e. treatment of the physically treated surfaces with caustic alkali and alkali zincate prior to the treatment with a strong mineral acid or acids, still further alloys are added to the field of the process. This last form of the process (i. e. physical treatment, treatment with caustic alkali and zincate, treatment with mineral acid, and then a second treatment with caustic alkali and zincate) appears to be capable of preparing substantially all the common commercial aluminum alloys for the reception of electroplates, and especially for the reception of electroplates of chromium. It will be understood that no preparatory or intermediate plating step is required with my present invention, at least usually. That is to say, after such preparatory steps as indicated above the metal desired for the plate is electrodeposited directly on the aluminum article, at least usually. Also usually each treatment, or at least each chemical treatment, is followed by a water rinse to wash away'matte'r that may be adhering to the surface as is customary in plating practice, including, usually, a water rinse after the final treatment with caustic alkali and alkali zincate solution and prior to electrodepositing the metal of the plate that is desired.

The steps or treatments described above may. follow one another immediatelyfor with only such little delay between each two as is consistent with convenient shop practices. On the other hand, after the rinsing which foliows the physical treatment the treated article can be set aside many hours, if desired, say up to twentyfo'urho'urs, before being subjected'to the next step or treatment which, of course, may be either analkal'i zin'c'ate o r acid treatment). Likee wise after the rinsing following an alkalizincate treatment, the next'treatment (either an acid treatment or the electroplating) can be delayed many hours. Also a delay may be interposed between the rinsing which follows the completion of theacicitreatment and the subsequent alkalizincate treatment, but in this case the permissible delay is generally shorter; say up to one hour. During such intervals the aluminum can be allowed to, stand in the open air. However if the atmosphere contains considerable dust or other foreign matterlthat tends to, deposit on the alu-v minum, the permissible intervals between steps or treatment are shortened thereby. Any oxide that mayform' on the, treated aluminum surfaces duringsuch intervals between treatments seems to have no detrimental effects.

Now taking up. the foregoing matters in greater t i -i,

As'f before indicated, usually it is necessary or e fable o, ai an cr e we uminu bfioire'undert'aking plating operations in order to vethsfseiram grease that have accumunit rom ha snag e previous manufac Them'anne'r in which this is a iee QneraHv. a r .I HPh smtoduce o cial change in the aluminum surgieas uf 'g by. the commonly used vapor deg easing operations wherein the articles are exposed to thev apors of highly volatile solvents, e791! Q S. i th ethyle ol chl r d u as ura? lsh p t da tr ch orct ene, perchlorethylenie andv the, like, is satisfactory. Usually at ea... he. a uminum. art e is i se with water after the degreasing and cleaning.

The aluminum articles are then ready for the operations ofthe invention. As before indicated, the first step of the, invention is, or includes, a physical; treatment which may be. by. vapor blasting and preferably is, by, vapor blasting. That is, the whole or such parts or. areas of the articles as are. to, receive the. electroplate are subjected to vapor, blasting. until the. surface film of adherent aluminum oxide on these parts or areas has been thoroughlybroken down and an apparently new surfaceexposed, preferably finely roughened. For example, the roughe'ningv may be such that its individual. depressions arev invisible to the eye, but. under a magnification ofthe order of five hundred diameters or more the surface seems to 4 consist of small irregularly shaped and irregu larly overlapping pits or other indentations. Such a blasting effect can be obtained, for example, by using as the grit the crushed siliceous rock of Arkansas known as Novaculite, the grit being of 100 mesh size (which grit has diameters ranging from about 0.0005 to about 0.03 inch), r 1 m h si e (grit anging i d amet from about 0.00015 to about 0.00025 inch), or of an intermediate size, or mixtures of such grits, and being suspended in a liquid, for example water (say from 4 to 5'pounds of the grit to each gallon of the liquid), and the mixture being driven toward the aluminunrfrom an internally-cylindrical nozale having an orifice about one-quarter inch in diameter delivering the mixture at an angle of about 45 to to the area or parts to be 'plated at a distance of about one-half inch or. more therefrom, the mixture being delivered to the nozzle under a head of about 4 feet, and sufficient air being supplied to the liquid passage leading to'the nozzle, ata pressure of about 100,. pounds per square. inch, for delivery from the nozzle with the liquid-grit mixture, to discharge. about three-fourths of a gallon of the liquid-grit mixture from the nozzle per minute, and continuing such blasting until the initial oxide film that is on the area of parts to be plated is completely broken down and'an apparently new and'finely roughened surface. produced or developed. Eurther, a suitable degree of'blasting can: be deter-. mined by the action of the blasted surface in an alkali-zincate solution such as that here ole-f scribed' I. e. if submerging the blasted surface in the alkali-zincate solution results in no or only. a small evolution of gas and ajsubsta ntially imm t deposit lit s rthi w le Suriac the l t a be nifiq sm an not. c ve; on h e h n if t e as. evolu ion. ro u e or h 9 oa in ai dow n. the. last d.- ce i P9 79 1 icros o a l the l tin s. improper er h ur ce ar a to. be. e te i a th cl ma be move o t transversely ofthe mixture. stream during the blasting so as to subject the. various parts of a r e. are o. he stream. om, the nozzle intert ntl or ore than e n zle. i char in to different parts of the article, may be used simultaneously, or both, and thereby, the time required for the necessary. degree of vapor blasting shortened, accordingly, Obviously the, time. required to produce, a new, surface by. such means in any instance will depend on the size of the area or areas bein treated, the number-of nozzles employed, etc. However theforegoing. is subject to great variation, andaccordingly. this. example is to. be regarded only as. illustrative. ofthe. type of physical treatment that I prefer and a procedurethat produces a surface that is illustrative of the. type. of surface. here concerned. Any physical treatment that producesa surface of the same type. can. be. substituted.

Usually the aluminum is rinsed immediately after the physical treatment, say water, to remove. any. remnants of the treatment that may remain on it.

Generally the alkali and zincate solution (and each ofthem, if more than'one alkali and zincate treatment is employed), is an aqueoussolution, and conveniently the zincate has the same alkali metal as the caustic alkali. Any of the caustic alkalis can be used, such as caustic soda, caustic potash, and the like. I prefer sodium hydroxide. The concentration of the caustic alkali is sufficient to retain the zincate in solution'during storage, and usually to this end is a number of times that of the zincate. The concentrations of the two are such that the solution acts to remove substantially whatever remnants, if any, of aluminum oxide film that otherwise might remain on the surface of the article after the physical treatment or any treatment, whatever it may be, that precedes the alkali-zincate treatment, and also if the interval between an alkali-zincate treatment and whatever treatment may precede the alkali-zincate treatment is sufiiciently long to permit the formation of oxide on the aluminum anew, to remove this fresh oxide, and additionally deposit an extremely thin, relatively uniform layer of zinc on the surface, by displacement from the zincate. The interval, if any, between the completion of the preceding treatment and beginnin of the alkali-zincate treatment should not be longer, at most, than sufficient to permit the formation of a thin film of oxide of substantially uniform thickness on the aluminum. Speaking generally, the solution may contain from about thirty (30) parts to saturation of the caustic alkali, from about six (6) parts to saturation of the zincate, and sumcient water to make up 100 parts, all by weight. A good solution is one having thirty-five (35) parts of caustic soda (NaOI-I), seven (7) parts of sodium zincate (NazznOz) and fifty-eight (58) parts of water, all by weight. The treatment of the aluminum articles may be hosin spraying or otherwise; usually I prefer immersion of the article in the solution as a separate step after rinsing of the article following the precedin step whatever the latter may be. The treatment is continued until substantially the whole of the blasted surface has been covered with a coating of zinc, which may be very thin. With the 35-7-58 solution, immersion for about two minutes at room temperature is sufficient, at least usually. However the immersion time is not critical and can be varied, as will be understood from the foregoing. Sometimes it may be even as little as about one-half minute, and in some circumstances even as short as ten seconds, and it may be for materially longer than two minutes. Generally operation at room temperature appears to be satisfactory, especially with immersion as long as two minutes.

With some aluminum alloys, as indicated above, satisfactory adhesion or bond between the chromium and the base metal is obtained by the physical treatment and a single treatment with such an alkali and zincate solution. An example of this type occurs with the die casting alloy identified above as 380. First cleaning thoroughly, then rinsing with water, and then vapor blasting this aluminum alloy with 1250 mesh Novaculite grit as described above, then immediately water rinsing the article, and immediately thereafter or later immersing for two minutes at room temperature in a solution consisting of thirty-five (35) parts of sodium hydroxide, seven (7) parts of sodium zincate, and fifty-eight (58) parts of water, all by weight, then water rinsing again, and then chromium plating according to ordinary chromium plating practices, gives a good plate and a good bond between the chromium and the alloy. The chromium may be plated, for example, from an aqueous solution made up primarily of about thirty-four (34) ounces of chromic acid (C103) and thirty-four onehundredths (0.34) of an ounce of the sulphate radical (S04) per gallon and with about one (1) ounce of the chromium in trivalent form, at a temperature of about 130 F. and with a current density of from about one (1) to about six (6) amperes per square inch, using a customary lead anode.

As another example, when any one of aluminum alloys has been plated with chromium and thereafter the plate stripped from the alloy by electrochemical action (i. c. with the plated article as an anode in an electrolyte consisting of a solution of, say, from fifteen (15) to twenty (20) parts of 66 B sulphuric acid and from eightyfive to eighty (80) parts of water, by Weight), the alloy can be replated with chromium with a good strong bond by the same two-step preparatory procedure. With this two-step preparatory operation however, it seems to be more difiicult to obtain a good bond when the blasting is done with 100 mesh Novaculite grit than when done with 1250 mesh Novaculite grit. Accordingly with the two-step preparatory procedure I prefer to use a grit finer than 100 mesh and usually I use the 1250 mesh Novaculite or thereabout.

The concentration of acid in the mineral acid solutions is variable over a wide range. For example where the acid is nitric (HNOs) the acid solution may be between about five (5) parts of the 42 B. nitric acid to ninety-five parts of water and about seventy (70) parts of the 42 Be. acid to thirty (30) parts of water, all by volume. The nitric acid mentioned throughout this specification is 42 B. acid. However when the lower concentrations, i. e. five (5) parts and somewhat higher, are used on copper-containing alloys, the solution tends to become contaminated with copper rather rapidly so that in even such short periods of continuous use as three or four days, the copper concentration may become so high that on occasion copper will deposit out of the acid solution by displacement onto an article being treated. Such a deposit tends to cause a poor bond between a subsequently applied chromium plate and the aluminum surface. Further, if the acid solution is too dilute, it tends to pitetch the aluminum if the aluminum is subjected to the solution too long, and during the subsequent treatment of the article with an alkali and zincate solution the zinc tends to build up to too thick a coating, and this also tends to a poor bond. Likewise at the other end of the range, too high a concentration of nitric acid tends to poor results at times at least. For example, when an acid treatment is preceded by an alkali and zincate treatment, an acid concentration of about sixty-five (65) or seventy (70) parts tends to cause the formation of blackish streaks on at least some of the alloys, and the bond between a chromium plate and the aluminum, at the areas occupied by these streaks, tends to be poor. Generally therefore, both to obtain uniformly good results and to avoid having to discard and. replace nitric acid bath too frequently I use in 11 acid baths a nitric acid concentration appreciasly higher than ten 0) parts and less than seventy (70) parts, advantageously somewhere between fifteen (15) parts and sixty-five (65) parts, and preferably somewhere around twenty-f1ve (25) parts, the remainder to make up a total of one hundred parts in each instance being water, all by volume. Usually an aqueous solution of about twenty-five (25) parts of nitric acid to seventy-five (75) parts of water, with occasional relenishments of acid and water (usually mostly acid, because drag-in of water from the previous rinsings tends to maintain sufficient water in the solution), can be used for quite a period (say up to five or six Weeks of forty working hours each) of water, by volume, at room temperature, for about one (1) minute, then rinse it with water, and then immerse it in a solution composed of twenty-five (25) parts of 42 B. nitric acid and seventy-five (75) parts of water, by volume, at room temperature, for about thirty (30) seconds or at least until any zinc there may have been on the article has disappeared. On the completion of either of these acid treatments, again rinse the article with water, then immerse the article in a solution of sodium hydroxide and sodium zincate in water such as that described in the first example above of plating on alloy 380, at room temperature, for two minutes or until a new film of zinc has been deposited on the article, and then again rinse with water. The article is then ready for plating, and can be plated with chromium, e. g. by the plating procedure of the former example.

It will be understood of course that the invention is not limited to the specific details appearing above except as appears hereafter in the claims.

What I claim is:

1. The method of preparing an aluminum article for electroplating which comprises vapor blasting that portion of the article that is to be plated, with a mixture of air, water and abrasive particles and thereby breaking down the adherent film of oxide thereon to expose a new surface,

the abrasive particles being of from about one hundred mesh to about twelve hundred and fifty mesh, and subjecting said portion of the article to the action of caustic alkali and alkali zincate to produce a deposit of Zinc on said portion of the article.

2. The subject matter of claim 1, characterized by the fact said portion of the article is subjected to the alkali and zincate after the completion of the blasting operation.

3. The subject matter of claim 2, characterized by the fact that directly following the blasting operation, and prior to the subjection of the article to the alkali and zincate, the portion of the article to be electroplated is subjected to at least one solution of at least one strong mineral acid.

4. The subject matter of claim 1, characterized by the fact that said alkali and zincate operation i performed subsequent to said blasting operation, thereafter the portion of the article to be electroplated is subjected to at least one solution of at least one strong mineral acid, and thereafter said portion of the article is subjected again to the action of caustic alkali and alkali zincate to produce a deposit of zinc on said portion of the article.

5. The method of producing a deposit of metal on aluminum which comprises treating the aluminum in accordance with claim 4, and thereafter electroplating the metal on the portion of the aluminum that has been so treated.

6. The subject matter of claim 1, characterized by the fact that in addition to subjecting the area to be plated to blasting and to alkali and zincate, said portion of the article, subsequent to the blasting, is subjected to the action of at least one solution of at least one strong mineral acid and the last significant step in the preparation for electroplating consists in subjecting said portion of the article to the action of an aqueous solution of caustic alkali and alkali zincate.

7. The method of producing a deposit of metal on aluminum, which comprises treating the aluminum in accordance with claim 1, and thereafter electroplating the metal on that portion of the aluminum that has been so treated.

KUNO L. VAN DER HORST.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES The Monthly Review of American Electroplaters Society, March 1946, pages 269, 270, 273,

Metal Finishing, February 1949, pages 48-53. 

1. THE METHOD OF PREPARING AN ALUMINUM ARTICLE FOR ELECTROPLATING WHICH COMPRISES VAPOR BLASTING THAT PORTION OF THE ARTICLE THAT IS TO BE PLATED, WITH A MIXTURE OF AIR, WATER AND ABRASIVE PARTICLES AND THEREBY BREAKING DOWN THE ADHERENT FILM OF OXIDE THEREON TO EXPOSE A NEW SURFACE THE ABRASIVE PARTICLES BEING OF FROM ABOUT ONE HUNDRED MESH TO ABOUT TWELVE HUNDRED AND FIFTY MESH, AND SUBJECTING SAID PORTION OF THE ARTICLE TO THE ACTION OF CAUSTIC ALKALI AND ALKALI ZINCATE TO PRODUCE A DEPOSITE OF ZINC ON SAID PORTION OF THE ARTICLE. 